1. Field of the Invention
This invention relates to integrated circuit manufacture and more particularly to a method for measuring dopant profile characteristics within a semiconductor substrate.
2. Description of the Relevant Art
Manufacture of an integrated circuit begins by diffusing or implanting with impurity ions one or more isolated regions across a semiconductor substrate. When activated through temperature anneal, the implanted regions become conductive. Deposited on the upper surface of one or more conductive regions is a plurality of interconnect conductors. The implanted regions and overlying interconnect form a monolithic integrated circuit well known in the art.
Modern integrated circuits employ densely patterned electronic devices patterned across the semiconductor substrate. Each device is quite small and, in some instances, is less than only a few square microns in area. Smaller devices dictate small dopant regions closely spaced across a majority if not the entire wafer. The dopant regions can be formed in any type of semiconductor substrate including, for example, silicon and gallium arsenide. The dopant regions are formed using either n-type or p-type impurities such as, for example, boron, phosphorus, arsenic, antimony, boron fluoride, etc. The dopant regions are formed by actively injecting or passively diffusing a desired impurity into one surface of the substrate. Modern fabrication techniques generally use an ion implanter for actively injecting impurities into the dopant regions. Medium to high energy implanters can easily implant the impurities through a thin oxide naturally formed or purposefully placed on the substrate upper surface. If the substrate is silicon, then a native oxide occurs when the substrate surface is exposed to oxygen. In many instances, the substrate surface is purposefully allowed to grow a thin gate oxide in addition to or in lieu of native oxide.
The electronic devices formed on the substrate surface are generally classified as passive or active devices. An active device generally includes a transistor, whereas a passive device generally includes a capacitor or resistor. Whether active or passive, functionality thereof is determined by many factors. An important factor is the concentration of dopant atoms existing within the dopant regions. To determine resistivity of the dopant regions, turn-on characteristics of a transistor channel between dopant regions, parasitic capacitance at the junction between dopant regions and substrate, susceptibility of hot carrier injection from dopant regions, as well as other characteristics, it is necessary to measure concentration of dopant/impurity atoms in the dopant regions. Periodic measurement of the dopant regions allows engineers and scientists to predict operability of the ensuing device.
There are numerous ways in which to characterize the dopant region. One way is to profile a cross-section of the wafer severed along the dopant region to expose that region extending from a point just below the surface oxide a distance toward the wafer backside surface. The dopant region profile (referenced hereinafter as "dopant region") therefore has an exposed cross-section surface extending perpendicular (i.e., vertically into the substrate from the oxide) and lateral (i.e., horizontally) to the oxide. Measuring dopant concentration, therefore, begins by measuring concentration in two dimensions along both the vertical and lateral directions.
Determination of two dimensional impurity concentration along the dopant region has evolved over the years. Early measurement techniques generally measure resistance and convert each resistance reading to a concentration amount. Resistance-to-concentration conversion requires well known spreading resistance probe (SRP) measurement and conversion techniques. SRP uses a pair of probes brought in contact with the surface of the dopant region. A voltage of several millivolts is applied across the probes and a resulting current is measured. A resistivity of the contacted dopant region can then be measured and related to concentration as a function of carrier mobility. While SRP techniques provide beneficial readings of dopant concentration levels at the surface of the dopant region, SRP techniques suffer many disadvantages. Firstly, SRP probes can only measure in a single dimension between probes the spreading resistance at the surface of the dopant profile. Secondly, and more importantly, failure of SRP probes to adequately contact the dopant region profile will deleteriously affect resistive readings. Thirdly, SRP measurements are limited by the resolution of the probe tip geometry and, since the probe tip diameter must be large to carry adequate measuring current and voltage, SRP measurements are generally limited to resolutions exceeding several microns.
Another widely used dopant characterization technique is secondary ion mass spectrometry (SIMS). SIMS can directly measure, albeit at low lateral resolutions, the dopant concentration in two dimensions not only at the surface region but as a function of depth. SIMS technique consists of using an ion beam (usually oxygen or cesium) to sputter away layers of the dopant region. The sputtered dopant region produces ions that can then be mass analyzed. Sensitivity of the mass analyzer is limited by interference between desired ions and ion complexes. This problem is particularly acute when measuring, for example, .sup.31 P and SiH complexes having somewhat similar atomic mass. A more important limitation of SIMS measurement is lack of resolution. SIMS resolution is limited by the diameter of the sputter beam impinging the dopant region. Generally speaking, SIMS sputter beam diameter oftentimes exceed one-half micron (.mu.m) thereby limiting SIMS usefulness to dopant region geometries which are on the order of one micron in lateral and vertical dimension. In order to determine a concentration gradient across a small dopant profile less than, for example, one half micron, a higher resolution measurement technique is needed.
Responsive to the need for higher resolution, many researchers have discovered the interrelation between dopant selective etching and topography profiling. In an article to Takigami, et al., "Measurements of the three-dimensional impurity profile in Si using chemical etching and scanning tunneling microscopy", Appl. Phys. Lett. 58 (20), May 20, 1991, researchers have pointed to the advantages of using an etchant whose etch rate depends upon impurity concentration for removing a portion of the dopant region profile. Takigami, et al. describe the importance of cleaving the dopant region to obtain a dopant region profile, etching the profile with a concentration-dependent etchant, and then measuring the resulting upper topography with a scanning tunneling microscope (STM). The etchant is capable of removing the doped substrate with high resolution demarcation, and the STM is capable of measuring the resulting topography also at high resolution.
The principal of STM operation generally comprises an atomically sharp tip brought near enough to the dopant region surface that the vacuum tunneling resistance between the surface and tip is finite and measurable. The tip scans the surface in two dimensions, similar to a raster pattern, while the height is adjusted to maintain a constant tunneling resistance. In order to maintain the constant tunneling resistance, the metal tip is displaced by a feedback voltage read from the tip to various piezo drives connected to move the tip. The piezo drives move the probe tip toward or away from the surface as it is being scanned across the surface to yield a contour map of the surface.
STMs in combination with concentration-dependent etching provide advantages of higher resolution profiling of the etched surface. Once the topography is known, it is then related to an impurity concentration using a known, unetched region under, for example, a protective photoresist layer. The unetched region being used as a calibration surface is generally located on the surface of the wafer and not on the scanned, profile area being measured. Comparison between dissimilar surfaces prepared under dissimilar conditions is an inappropriate way in which to calibrate a test procedure. The problems of using a dissimilar surface as the calibration surface are manyfold. Firstly, use of a photoresist material to protect the calibration surface renders caustic material on the wafer, which in some instances cannot be removed at the to-be-measured, critical cross-section. Secondly, any photoresist left on the calibration surface or the cross-section surface can skew the high resolution STM profile, thereby defeating the purpose of STM.
Related to the problems of preparing a surface in which to calibrate impurity concentration to etch depth of the dopant region, STM simply cannot measure all types of dopant profiles. The dopant region (i.e., dopant profile) generally includes various dielectric and conductive cross-section layers. Dielectric and conductive layers are formed during wafer fabrication on one surface of the wafer on which a cross-section is thereafter exposed for concentration readings. STM, due to the necessity for having two conductors between the tunneling path, can only measure the topography of a conductive surface. The dielectric oxide adjacent the dopant profile cannot be detected by the STM probe. Since tunneling current does not exist between the probe and the oxide, the probe can inadvertently contact the dielectric and possibly break upon contact. STM is therefore of limited use when measuring a dopant region formed according to normal processing steps having dielectric layers exposed in the profile being measured.
In an effort to overcome limitations of STM, atomic force microscopy (AFM) was discovered capable of measuring topography of both conductors and dielectrics. Raineri, et al., "Carrier distribution in silicon devices by atomic force microscopy on etched surfaces", App. Phys. Lett. 64 (3), Jan. 17, 1994. Raineri, et al. describe AFM used to profile a dopant region etched in accordance with dopant concentration. While AFM profiling has numerous advantages, it is nonetheless limited by the calibration technique used.
In essence, a benchmark etch depth must be correlated to an impurity concentration. The benchmark etch depth and corresponding impurity concentration must be formulated from a calibration surface. Given a benchmark etch depth and corresponding impurity concentration, any etch depth reading in a to-be-measured (i.e., target) dopant region about the benchmark can then be correlated to an impurity concentration value. To obtain reliable target readings, it is therefore necessary to maintain the integrity of the calibration surface. It is also necessary that the calibration surface be prepared under similar processing constraints as the target surface being measured. Thus, it would be desirable to form a calibration surface having a dopant concentration profile similar to the target concentration profile. If the target and calibration surfaces are not flat and are not: prepared under similar processing constraints, then AFM readings on the target region will not calibrate back to an accurate impurity concentration derived from the calibration surface.
Of further importance in calibration is the need for establishing an initial scan reading location consistent on both the calibration and target surfaces. It is desirable that the scanning of both the calibration and target surfaces be initialized at a known location identical with each other. Further, the reading location must be formed such that scanning of the AFM probe and readings therefrom are not compromised by large disparities at the upper surface being scanned. It is therefore essential that a reading location adjoining both the calibration and target dopant profile be properly fashioned to minimize disparity of the upper scanned surface and to maximize measurability with the highest possible detection resolution.